This invention is related to the pathological condition of arthritis. Arthritis is the wear and tear of the articular surface of the joint. “Arthro” means joint; “Itis” is inflammation. Normally the bone ends at the joint surfaces are covered with articular cartilage. This type of articular cartilage with which we are born is called hyaline cartilage and is smooth, is lubricated by synovial fluid, and in some people will wear more than a hundred years. In others, depending upon the quality of the cartilage which they have inherited, it will wear down exposing bone surface which then rubs against bone surface producing inflammation of the joint and thus the arthritis.
The most common form of arthritis is osteoarthritis which is the result of wear and tear of the cartilage with time and use. The cartilage may also be destroyed by a systemic disease such as rheumatoid arthritis or gout. The smooth, matching congruous articular surface may also be disrupted by trauma, either a fracture that goes through the joint surface, or an infection wearing away the cartilage. Nevertheless, the end result is the exposure of bone against bone which is painful, disabling and limits function.
Once the original hyaline cartilage with which we are born is destroyed, the joint is incapable of stimulating the regrowth of hyaline cartilage or growing any other type of articular cartilage. Therefore, various operative procedures are used to alleviate the pain and dysfunction associated with arthritis; these include fusion of the joint, thereby relieving the pain by completely stopping motion at the joint, and some type of arthroplasty reconstruction of the joint. This may be done by inserting some prosthetic material between the joint surfaces; a biological tissue, such as tendon graft or a strip of fascia, which is the tissue covering the muscle sleeve, may also be placed between the joint surfaces. There are obviously disadvantages to each of these alternatives as well as advantages. A fusion will result in permanent loss of motion. The biological tissue which is inserted will often degenerate into scar tissue and often will not wear well with stress. Various types of prosthetic implants are used, some more advantageous than others.
The carpometacarpal joint of the thumb, also called the basal joint of the thumb, is the only joint of the hand that we rotate. The joint is essentially saddle shaped, having not progressed in evolution as quickly as man has. Man is the only primate on earth that does rotate the thumb into opposition against the fingertips; therefore, we are constantly turning in the saddle. Depending upon the quality of cartilage, which is inherited, the stress to which it is often subjected will often cause the articular cartilage to wear down producing painful, grinding arthritis. Almost 10% of the population will be subject to this particular pathological entity.
Over 25 years ago, Dr. Charles R. Ashworth and I developed an arthroplasty for this joint in which we would use a wafer or disc-like implant made of a silicone rubber sold by Dow Corning as Silastic. Silastic has proven to be one of the most durable and best tolerated implant materials in the upper extremity and is often utilized for major joint replacements at the knuckle joints, the joints in the middle of the fingers, at times between the small ball-bearing or carpal bones of the wrist, and as stated, at the base of the thumb. We have used this Silastic interposition arthroplasty quite successfully for over 25 years. FIGS. 1A–1C illustrate treatment of arthritis at the scapho-trapezial-trapezoidal joint 2. FIG. 1A illustrates a line of resection 4 of the convex dome of the scaphoid. FIG. 1B shows the status following resection of the arthritic surface, including the preparation of a cylindrical recess 6 formed in the center of resected surface 8 for receipt of a stem 10 of an implant 12 made of Silastic. FIG. 1C illustrates the implant 12 in place. FIGS. 1D–1F illustrate a conventional treatment of an arthritic carpometacarpal joint 3 of the thumb. FIG. 1D shows the amount of the saddle or convex portion 5 to be resected from the trapezium 7. FIG. 1E shows the resected trapezium 7 prepared for accepting a hat-shaped implant 9; implant 9 is typically made of Silastic material. FIG. 1F shows implant 9 in place between the resected trapezium 7 and the base of the metacarpal 11.
While these procedures have been quite successful and have improved the quality of life for numerous people over the last 25 years, there have been some recent questions regarding the efficacy of continued use of any implant made of silicone rubber. Therefore, I gave thought and consideration as to what other type of material might be used as an interpositional implant.
As background to the development of the present invention, I conducted a review of reported cases of patients who have broken or worn away their Silastic implants. The joints of these patients, based on re-exploration conducted from a period of nine months to several years postoperatively, have been covered by a smooth, white, moistened, glistening surface in the area where there had previously been raw resected bone. Therefore, my postulation is that the original surgery, removing the arthritic spurs and the uneven joint surface, left a raw bony surface normally covered by a blood clot or hematoma which then came in contact with the inert, smooth Silastic disc. The gradual fibroplasia of the blood clot appears to progress to fibrocartilage. At times the surgical exploration of this surface tissue has been biopsied and has microscopically proven to be fibrocartilage.
Another important concept is the body's attempt to repair the joint surface with normal scar tissue. The basic cell of healing is called the fibroblast and will develop at any site of injury and is the transformation of a normal blood clot. The fibroblast will go through a series of histological microscopic changes called fibroplasia. The fibroblast on a surface where there is constant motion will change and develop into an entity known as fibrocartilage. This is white, smooth and looks very much like cartilage. It does not, however, have a basic property of hyaline cartilage, that is the ability to withstand impact stress. Therefore, fibrocartilage has been known to not survive the weight bearing stress of lower extremity joints, including the hip, the knee or the ankle. There have apparently been no studies of the competence or resilience of fibrocartilage as a joint surface for non-weight bearing, upper extremity joints.